The maintenance of genome integrity is essential for the accurate transmission of genetic information and to prevent tumorigenesis. Indeed, both DNA sequence and chromosome instabilities are associated with a high percentage and wide variety of human cancers. The repair of chromosomal damage must take place in the context of chromatin structure, but virtually nothing is known about the interactions of the repair machinery with the components of chromatin. We have discovered that the N-terminal domain, or "tail", of histone H4 is required for efficient DNA damage repair and that this function requires the activity of an acetylatable lysine residue. On the basis of genetic and biochemical observations, we hypothesize that H4 facilitates DNA damage repair at double-strand chromosome breaks by interacting with components of the repair machinery through protein-protein interactions signaled by lysine acetylation. We further hypothesize that specific histone acetyl transferases and/or histone deacetylases participate directly in DNA repair via reversible H4 acetylation. To test these models we will focus on three major research questions. First, we will examine the structure, processing and fidelity of repair at double-strand breaks in histone H4 mutants defective for reversible acetylation. We will test the role of H4 acetylation in both nonhomologous DNA end joining and homologous double-strand break repair. Second, we will examine the genetic and biochemical interactions between histone H4 acetylation and DNA repair proteins. We will exploit our finding that histone H4 acetylation site mutants are hypersensitivity to the radiomimetic drug camptothecin to screen for interacting genes, and we will purify yeast Ku70-associated proteins directly to identify new components of the H4-dependent repair pathway. Finally, we will use genetic screens and in vivo cross-linking coupled with chromatin immunoprecipitation to characterize a specific histone acetyl transferase for its role in DNA damage repair and its influence on the state of histone H4 acetylation at a defined double-strand break. Our observation that reversible histone H4 acetylation is required for DNA repair represents a previously unrecognized role for this important chromatin modification.